A magnitude 5.3 aftershock hit China's Sichuan Province Thursday, raising the risk of flooding in towns already devastated by last month's earthquake. Since the May 12 quake, the region has experienced hundreds of tremors as well as a major false alarm that caused thousands to flee their homes. Is it possible for an aftershock to be as large as the main event?
By definition, no. If an earthquake is followed by a more powerful seismic event, it's automatically redefined as a foreshock. The largest tremor is always classified as the earthquake; everything else is either a foreshock or an aftershock. It's also highly unlikely that a quake as massive as the one in Sichuan, which registered a magnitude of 7.9 on the Richter scale, would be followed by an event of the same intensity—especially after so much time had elapsed.
There are two principles that describe the typical behavior of aftershocks. The first, called Omori's Law, predicts that most shocks will occur immediately following the earthquake and become less and less frequent over time. The other, known as Båth's Law, states that the largest aftershock is, on average, about 1.2 magnitudes smaller than the main quake. (The Richter scale is logarithmic, meaning that an earthquake measuring magnitude 6.0 is 10 times larger than one measuring 5.0.) Seismologists have begun using these rules to create maps of California that forecast where an aftershock is most likely to occur. But as several news reports noted after a Sichuanese TV station issued an erroneous warning a week after the quake, there is still no reliable way to predict the exact timing or location of an aftershock.
A 2002 study (PDF) of 117 earthquakes with magnitudes of 7.0 or higher found that only 13 were followed by nearby quakes that were at least as powerful. But whether these 13 should be classified as foreshocks or as "triggering earthquakes" is a matter of some dispute among seismologists. Traditionally, foreshocks and aftershocks have been defined as events occurring one or two "fault lengths" away from the main quake, while triggering and triggered earthquakes are separate events that occur at a more remote distance. After the magnitude 9.1 quake in Sumatra that caused the 2004 tsunami, scientists debated whether a subsequent shock three months later—which the U.S. Geological Survey classified as the seventh-largest earthquake since 1900—was an aftershock or a separate quake along the same trench.
Aftershocks become less frequent over time, but they can still occur years—or perhaps even decades—after an earthquake. Some researchers even believe that seismic activity in the Midwest—including a minor earthquake this April—is the belated result of a massive 1811-12 temblor centered in New Madrid, Mo. Such late aftershocks are more likely to occur when a fault exists in the middle of a tectonic plate; the Longmen Shan fault in Sichuan, on the other hand, is located at the collision of India and Eurasia.
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Explainer thanks Karen Felzer and Tom Parsons of the U.S. Geological Survey, Tom Jordan of the Southern California Earthquake Center, and Seth Stein of Northwestern University.